Method of opacifying the surface of glass articles



Patented Sept. 8, 1953 UNITED STATES PATENT OFFICE METHOD OF OPACIFYING THE SURFACE OF GLASS ARTICLES Stanley Donald Stookey, Corning, N. Y., assignor to Corning Glass Works, Corning, N. Y., a corporation of New York No Drawing. Original application July 7, 1950, Serial No. 172,596. Divided and this application May 16, 1952, Serial No. 288,313

2 one alkali metal oxide as indicated, and alumina in the amounts specified. The use of'these ingredients in proportions outside the indicated ranges results in an unsatisfactory product. For example, an excess of S102 or a deficiency of alkali metal oxide produces a glass which is difficult to melt and which tends to spontaneously gold as the primary photosensitizing agent, aredescribed in my pending application Serial No. 69,769, filed January 7, 1949, now Patent No. 2,515,943, issued July 8, 1950. For successful development, such gold-containing glasses must .be held for extended periods of time at temperatures close to their softening points. Because of their resulting tendency to soften, these glasses have not been entirely satisfactory for the production of all types of ware, however, and close temperature control must be employed to prevent undue deformation of such ware as can be readily made. (As used herein, the softening point of a glass is that defined in an article entitled A method for measuring the softening temperature of glass, by J. T. Littleton, Jour. Amer.

Ceram. Soc., 10, 259 (1927).)

I have now discovered photosensitively opacifiable glass compositions which avoid these difficulties. These glasses employ silver as the primary photosensitizing agent and are particularly advantageous in that they can be satisfactorily developed in considerably shorter times and/or at considerably lower temperatures, whereby softening and resulting deformation of were can be avoided. Moreover, the present glasses possess a greater degree of photosensitivity.

The glasses according to the present invention comprise essentially, on the oxide basis, 55% to '7 5% SiO-z the indicated proportion of at least one alkali metal oxide selected from the group consisting of up to 2% H20, 5% to 18% NazO, and up to 13 K20, the selected alkali metal oxide including Na2O, the total alkali metal oxide content being 12% to 18%, 2% to 12% A1203, 0.0001% to 0.3% of silver computed as AgCl, 0.005% to 0.05% 0e02, 1.8% to 3.0% of analytically determined fluorine, and the indicated proportion of a halogen selected from the group consisting of 0.01% to 0.2% of analytically determined chlorine, 0.02% to 0.4% of analytically determined bromine, and 0.03% to 0.6% of analytically determined iodine. These essential constituents must amount to at least 85% of the total composition.

The base glass, exclusive of photosensitizing and opacifying agents, comprises silica, at least opacify during working. On the other hand, an

excess of alkali metal oxide or too little S102 ob- V jectionably decreases the chemical stability of the glass. At least 2% A1203 is necessary to prevent spontaneous opacification but more than 12% objectionably hardens the glass.

Preferably, the alkali metal oxide comprises Naz0 alone. L120 and/or K20 can be employed in combination with NazO in the indicated proportions, however. More than the indicated maximum proportion of Liz() tends to objectionably soften the glass and to result in devitri- I fication, while larger amounts of K20 cause the 7 glass to become too hard and tend to bring about spontaneous opacification.

Small amounts of B203 may advantageously be employed to facilitate meltin and working of the glass. However, amounts greater than 5% tend to weaken the photosensitivity of the glass and to cause it to be excessively soft.

The oxides of the divalent metals Be, Mg, Ca, Zn, Sr, Cd, and Ba may also be included within the limits specified below in order to generally improve the chemical durability of the present glasses. Because of their tendency to induce spontaneous opacificatiomhowever, BeO, MgO, and CaO should not be present in amounts ex-' ceeding 3% either individually or collectively; ZnO, SrO, and BaO in amounts up to 12%, and CdO in amounts up to 5%. Of these divalent oxides, ZnO has been found the most desirable.

Regardless of which divalent oxides are employed, however, the total content thereof should not be greater than 12%.

It is essential that the presence of materials which strongly absorb ultraviolet radiations be avoided. Such absorptive constituents include most glass colorants, particularly selenium or its compounds and oxides of iron, copper, uranium, and vanadium, as well as the non-coloring oxides of arsenic, thallium and lead.

The silver may be introduced into the batch as any salt or compound thereof. Preferably, a water solution of silver nitrate is employed. The minimum amount of silver effective to produce a noticeable degree of photosensitivity is 0.0001% computed as AgCl. The opacified image is gen erally white when the silver content is less than 3 about 0.002%. As the proportion of silver is increased, however, a yellowish tint tends to be imparted to the opaque image, particularly with long or intense irradiation. With a silver content in excess of about 0.05%, the image is almost invariably colored. More than 0.3% of silver tends s on a o s color the gla s rou hout during melting or reheating.

The presence of C602 is necessary in order to promote and enhance the photosensitivity of the silver. Less than 0.005% C602 is relatively inr efiective, while more than about 0.05% causes suflicient absorption of ultraviolet radiatigns to effectively destroy the photcsensitivity of. the glass. As a source of CeOz, a material known as cerium hydrate, having a cerium content equivalent to about 75% (2e92, has been satisfactory although other salts and cgrnpuunds of cerium may be used.

The amount of fluorine incorporated into the glass must be insufficient to cause opacification 3 3911 teheati is- The fluo ine. may he intro uc d into the batch as any Qf its common compounds, such sodium: or potassium-.silicofluoride, cryo: lls Qt alka i metal fluoride.

It is impossible, 2

to volatilize during melting; but I have found that the analytically determined amount remaining in the glass must not be less than 0.01% chlorine, 0.02% bromine or 0.03% iodine and not more than 0.2% chlorine, 0.4% bromine or 0.6 iodine. Smaller amounts than those indicated are ineffective to activate the silver, whereas larger amounts tend to cause spontaneous opaciication. These three halogens may be used collectively, if desired. When employed, however,- their total content, computed on a mole equivalent basis, must be within the limitations set forth above for any one taken individually.

The presence of up to 0.2% of antimony oxide computed as 51320:. or up to 0.1% SnOz is advantageous for increasing the photosensitivity of the lass. Gree e13 amounts of either oxide tend to gauge over ll coloration of the glass when melted, thus destroying the photosensitivity. SbzOs also exerts a desirable fining action during melt ing of the glass, and its presence is generally preferred to that of S1102.

The following batches are illustrative of glass compositions falling within the scope of my in.- vention (weight in pounds except the gold solu-.

however, to state a definite amount of fluorine tion, which is expressed in cc.)

A1(OH)3 Borax (anhydrous). 0210 Q3 Z110 Nal Corn Starch w ic n ntr u ed i to th batche the present gu set ll "produce the desired result nd all wa b u s a qqnsidete te m se has l st. b ols i ie tie n th m lti of i b ch. egass n soon the iim a f i re mi s fiee eP-Q th typ o melting "container?" that is, whether open or closed. I havenev'ertheless determined that the amount offiuorine in the final glass, n o 'a 'i' fi q tt q fluo i e u t. b not less than about 1 .8%; nor greater than about 3.0,%' to produce the present result, regardless of the. harititfi of fluorine added to the batch. Iiess han about 1.3% fluorine results in little or no opacification, while more than about fluorine'results in spontaneous opacification.

In what form the fluorine exists. in the unopacified', glass is not definitely known, but it is believed that at least some of it is in the. form ofallia i ietal'fiuoride dissolved in the glass. In any event,it has been determined that the opacifying' crystallites in the exposed and developed glasscoiitaififluorine and are composed largely, if not entirely, of alkali metal fluoride, in particular sodium fluoride.

The presence of chlorine, bromine, or iodine in the amounts. indicated necessary. to. act-i1 rate; the sil er "nd'make. it effectively photosem shi Lik uor ne; u e r e ee we The silver n t t is. incorpo ated in t e abov batches s a u e. queous oluti Ehs gold ut is epa ed, b d solvin me lic s ld n a ma in t e pr o ions "4 er -ms gold per cc. of solution.

As is w wn e exact om os ions oi l -con a n a ses c nn e. culat with accuracy from their batches. In the first la e. a n ed u bo e d rab e halo en in variable amounts is lost during melting and t ac halogen conte t of th n ass c be determined o ly b anal sis. M t et. he halogens are anions, but with which cation or cations they are combined is not definitely known. The calculatedfinal content of the halo,- ens c p ed s. Cl, B or I pe t ve y is therefore i d end nt. of th as om s ion, which is a wa Q mpu on th o i e. b is For practical purposes, however, the calculated percentages of theoxides and the calculated per-v centages of the halogens are generally computed together to a total of100,%, although this results in a slight error in the expressed amounts: of the various constituents ascomparedto their analytical amounts. The. calculated compositions of such glasses are therefore approximate.

The calculated composition of the, glass ob,- tained by melting batch 1, for example, is 633% SiO'z, 16.7% NazO, 10.1% AlzQc, !0 .6 B293,

0.0017 of silver computed as AgCl, 0.01% e02, 0.09% SbzOs, 2.9% F, and 0.25% Br. By comparison a glass produced by actually melting this batch in a large commercial melting unit at a top temperature of approximately 1400 C. contained by analysis 2.6% fluorine and 0.2% bromine. The composition of this glass as recalculated is then 69.6% S102, 16.8% NazO, 10.1% A1203, 0.6% B203, 0.0017% of silver computed as AgCl, 0.01% C602 0.09% 313203, 2.6% F, and 0.2% Br.

The glass produced by melting batch 1 possesses physical properties particularly desirable for melting and forming purposes and comprises a preferred embodiment of my invention. Due to the presence of ZnO, the glass melted from batch 2 possesses good chemical durability and represents another preferred embodiment. The composition of this latter glass, computed and recalculated in the manner described above, is 69.1% $502, 16.5% NazO, 6.6% A1203, 4.8% ZnO, 0.002% of silver computed as AgCl, 0.02% CeOz, 0.1% Sb203, 2.6% F and 0.2% Br.

It will be noted that all of the batches contain a mild reducing agent, the presence of which during melting is essential. When either a bromide or an iodide, such as NaBr or NaI, is used to furnish the halogen (other than fluorine), such compound itself functions as the reducing agent. If a chloride such as NaCl is used to furnish such halogen, then, except in the case of NHiCl, another material such as corn starch, as in batch 5, must be added.

In any event, the addition to the batch of a minor amount of a reducing agent other than a halide as indicated, preferably of an organic type such as starch, sugar, or urea, serves to increase the photographic speed of the glass by as much as five times. In general, corn starch in an amount up to about 0.2% on the basis of the glass or another reducing agent in an amount to provide the equivalent reducing power or effect is suiiicient for the purpose. Larger amounts tend to render the glass spontaneously opacifiable upon thermal treatment.

An oxidizing agent such as sodium nitrate is desirably employed with the larger proportions of silver, generally above about 0.05%, to insure that the silver dissolves in the glass. If the glass is also to contain gold as in batch 4, the use of an oxidizing agent is necessary.

The present glasses are clear and transparent when melted, worked, and cooled, and will so remain when merely reheated. Exposure to short-wave radiations, preferably those between 3000 and 3500 Angstroms, produces an invisible, latent image therein. This latent image, present only in the exposed portions of the glass, is converted to a visible, opaque image by subsequent heating. If a photographic negative or stencil is interposed between the glass and the source of radiation, only selected areas of the glass will be exposed, and the image formed will be a reproduction of the negative or stencil. The exposure time may vary from about ten seconds to an hour or longer, depending on the particular glass composition, the image characteristics desired, and the intensity and type of radiation employed. With a 60-ampere carbon are at a distance of one foot, an average exposure can be accomplished in about thirty seconds. Generally speaking, exposure times of over about 5 to minutes are unnecessary.

The latent image produced in the glass during by a two-stage development. The initial step involves heating the exposed glass for a time and at a temperature varying from about one minute at 50 C. above the softening point of the glass to about one hour at about 150 C. below the softening point. Temperatures lower than 150 C. below the softening point are ineffective, and temperatures higher than 50 C. above the softening point are both impractical and detrimental to the image. Preferably, unless a shaping operation is to be carried out simultaneously, the temperature should be about 50 to C. below the softening point to accomplish this initial development within a reasonable time and to avoid possible deformation of the article or sticking of the article to its support. Too rapid heating of the glass above 500 0. tends to destroy the latent image before itcan be developed and should be avoided.

This initial heat treatment does not cause any opacification but is nevertheless essential for the ultimate development of the latent image. If, however, the silver content is above about 0.002% as indicated above, the latent image may, as the result of such heating, develop into a real image which tends toward a yellowish tint but is otherwise transparent. It is believed that during such heat treatment submicroscopic nuclei of colloidal silver are formed in the irradiated portions of the glass and that, if the silver content is above about 0.002%, the silver nuclei will be of a size and number suflicient to tend to cause a yellowish coloration of the .glass.

Following such preliminary heat treatment the glass is cooled to a temperature below about 500 C. During this step, also, no further visible change in the glass occurs. It is believed that submicroscopic nuclei of the opacifying agent or agents, that is, alkali metal fluorides, are formed on the silver nuclei as a result of this cooling, and that the formation of such invisible fluoride nuclei is entirely dependent upon the presence of the silver nuclei without which opacification of the latent image could not be initiated. While the glasses must be cooled to below this temperature to accomplish the subsequent opacification, how much below seems to be immaterial and they may be cooled to room temperature if desired.

Having been so cooled, the glass is again heated at a temperature and for a time sufficient to cause the fluoride nuclei to grow and form opacifying crystallites. For this purpose the temperature should be not lower than about 100 0. below the softening point. The reheating time depends on the density desired in the final image and on the temperature, greater densities requiring correspondingly longer timesand higher temperatures shorter times. For

average image development, a period of 3 to 15- minutes is usually adequate. The finished glass: is thereafter cooled to room temperature.

It is only during this second heating that the final opacified image is developed and then only in the irradiated portions of the glass. Such image will appear white or uncolored unless the preliminary heating has caused development of a yellowish tint as described above. The opacity will be mor or less dense according to the nature of the exposure and the initial heat treat-- ment, which determine the number of nuclei formed, and also the final heat treatment, which determines the size to which the opacifying crystallites grow.

With some glasses containing fluorine in the upper halfor so of, the indicated range, it may be found that the initial heat treatment and intermediate cooling may be omitted and. that the desired opaque image can be obtained with a single heat treatment. In general, however, it will be found that the described two-step heat treatment is necessary.

Various effects can beachieved with th present glasses. For example, different shades or hues can be produced in the same article by exposing different areas to different. intensities. of irradiation 01' for different. times- Again, the temperature employed. in either heating step or the duration thereof has an efiect on the shade and/or hue produced. The use of pro grcssively slighter exposures together with progressively longer or higher-temperature heating in. thefirst heating. step, while keeping the term perature below 550 C. in the finalv heating step, results in the production of yellow, brown, orange, rose, purple, blue, and green hues in this order.

A further variation is obtained by the inclusion of SbaOs and SnOz, which together impart a grayish or a pinkish tintto the opal image depending on the proportions in which they are used. lhe total of. SbzOa and $1102. combined should not exceed 0.2%., and the maximum amount of S1102 should not exceed 0.1%. A pinkish coloration can also be produced by the inclusion of up to 0.01% of gold computed as Au. A colored background can be provided by including a suitable coloring material such as cobalt oxide or manganese xide in. the glass.

Further exposure of irradiated areas between the two heat-treating. steps surprisingly effects a reversal of the opal development, that is, the re-exposed areas remain clear and transparent following the second heat treatment. An overall initial exposure of an article followed by heating and subsequent re-exposure of a portion thereof thus produces a transparent image against an opaque background.

Surface opacity can be produced by special treatment in glasses of the present type melted from batches containing no silver. By applying. to the surface of such a glass a finely divided material containing silver or a compound of silver and heating the glass and the .material while in contact in accordance with. conventional silver-staining procedure, an exchange or silver for alkali metal in the glass is eifected. The surface f such a silver-stained glass, when subsequently held at a temperature above about 550 C. for about minutes or longer, and then cooled below about 500 C., becomes opacified upon reheating to a temperature not lower than about 100 C. below the softening point of the glass. Alternatively, the glass and the silver-staining material can be heated. to .a temperature above about 550 C. for at least 15 minutes, and the silver-stained glass then cooled and reheated as indicated. No exposure to short-wave radiations is necessary to produce such opacity. Reversal of this opal formation can be eifected by exposure of the stained area in the usual manner prior to the final heat treatment.

The .inclusion of corn starch in, the batch in amounts greater than that permissible for normal development procedures but not over about 0.6% on the basis of the glass, or of another reducing agent in an'amount to provide the-equiva lent reducing power, produces a surprising effect in that the tendency to spontaneous opacification upon heating is thereby reversed by exp sure of the glass to short-wave radiations prior to such heat treatment. Exposure of such a glass in selected areas followed by heating then renders the glass. Opaque in unexposed areas, but clear and transparent. in exposed areas. The thermal conditions for opal development in these glasses are identical with those of the final heat treatment of the developing process described above.

The present glasses are extremely useful in producing such completely opalized articles as tableware or incandescent lamp bulbs. Because of the variety of, effects obtainable with the present glasses, simultaneous production of alltransparent, all-opal and partially opalized ware from a single glass. melt is possible.

When the: present glasses are to be used in the manufacture of tempered opalware the article-can be inspected, prior to opacification and tempering, for stones. and other imperfections which would normally cause breakage. A par tially opacified article may also be tempered without setting up undue strains between the opacined: and transparent portions thereof. Thus for the first time a tempered article having bothtransparent and opaque areas can be produced. As is Well known, prior articles partially opacified by gradient heating become completely opacified' when heated for tempering.

Photosensitive glasses. containing from 0.05% to 0.3% of silver computed as AgCl are described and claimed in pending application Serial No. 51:3,4l, filed. December 8, 1943, by William H. nrmistead, now. Patent 2,515,936, issued July 18, 2.950. Such. glasses, when exposed and subse quently heated, develop a transparent yellow coloration in the exposed portions, while the unexposed portions remain clear and colorless.

in my pending application Serial No. 513,445, aisofiledDecember 8, 194.3,.now Patent 2,515,939, issued July 18, 1950, I have disclosed and claimed photosensitive glasses similar to those of Armistead but additionally containing sufficient fluorine to render them thermally opacifiab'le.

My pending. application Serial No. 695,801, filed September 9', 1946, now Patent 2,515,940, issuedJuly 18, 1 950, describes and claims photosensitively opacifiable glasses containing 0.025% to 03% of silver computed-as AgCl and 10% to 25% LigO, the amount of 1520' being suillcient to form lithium disilicate crystallites upon exposure and subsequent heating.

In my: pending application Serial No. 1,492, filed January 9, 1948, now Patent 2,515,275, issued July 18, 1950, there are described and claimed photosensitive glasses similar to those of Armistead but containing up to 0.1% SbzOs. Such glasses may also contain sulllcient fluorine to be thermally opacifiable.

I claim:

1. The method of opacifying at least a portion of the surfaceof a glass comprising essentially 55% to 75% SiO2, the indicated proportion of at least one alkali-metal oxide selected from the group consisting of up to 2% H20, 5% to 18% NazO, andup to 13% K20, the selected alkali metal oxide including NazO, the total alkali metal oxide content being 12% to 18%, 2% to 12% A1203, 0.005% to 0305% (3e02, 1.8% to 3.0% of analytically determined fluorine, and the indicated proportion of. .a halogen selected from the group consisting of 010.1% to 0.2% of analytical-ly determined chlorine. 0.02% to 0.4% of analytically determined bromine and 030.2% to 0.6% of analytically determined iodine, the essential constituents totaling at least 85 which comprises introducing silver in a concentration of from 0.0001% to 0.3% into the surface of the glass by bringing the glass and a finely divided material selected from the group consisting of silver and silver compounds into contact, and heating the glass and the silver-containing material while they are in contact to effect an exchange of silver for alkali metal in the glass, thereafter heating the glass to a temperature above about 550 C. for a period of at least 15 minutes, cooling the glass below about 500 C., and reheating the glass to a temperature not lower than about 100 C. below its softening point until the desired degree of opacity is developed.

2. The method as claimed in claim 1, which includes the additional step of exposing a portion of the silver-treated glass surface to shortwave radiations prior to the final heating step.

3. The method of opacifying at least a portion of the surface of a glass comprising essentially 55% to 75% SiOa, the indicated proportion of at least one alkali metal oxide selected from the group consisting of up to 2% LizO, 5% to 18% NazO, and up to 13% K20, the selected alkali metal oxide including NazO, the total alkali metal oxide content being 12% to 18%, 2% to 12% A1203, 0.005% to 0.05% 0e02, 1.8% to 3.0% of analytically determined fluorine, and the indicated proportion of a halogen selected from the group consisting of 0.01% to 0.2% of analytically determined chlorine, 0.02% to 0.4% of analytically determined bromine and 0.03% to 0.6% of analytically determined iodine, the essential constituents totaling at least 851%, which comprises introducing silver in a concentration of from 0.0001% to 0.3% into the surface of the glass by bringing the glass and a finely divided material selected from the group consisting of silver and silver compounds into contact and heating the glass and the silver-containing material while they are in contact to a temperature above about 550 C. to effect an exchange of silver for alkali metal in the glass, thereafter cooling the glass below about 500 C., and reheating the glass to a temperature not lower than about C. below its softening point until the desired degree of opacity is developed.

4. The method as claimed in claim 3, which includes the additional step of exposing a portion of the silver-treated glass surface to shortwave radiations prior to the final heating step.

STANLEY DONALD S-IOOKEY.

No references cited. 

1. THE METHOD OF OPACIFYING AT LEAST A PORTION OF THE SURFACE OF A GLASS COMPRISING ESSENTIALLY 55% TO 75% SIO2, THE INDICATED PROPORTION OF AT LEAST ONE ALKALI METAL OXIDE SELECTED FROM THE GROUP CONSISTING OF UP TO 2% LI2O, 5% TO 18% NA2O, AND UP TO 13% K2O, THE SELECTED ALKALI METAL OXIDE INCLUDING NA2O, THE TOTAL ALKALI METAL OXIDE CONTENT BEING 12% TO 18%, 2% TO 12% AL2O3, 0.005% TO 0.05% CEO2, 1.8 TO 3.0% OF ANALYTICALLY DETERMINED FLUORINE, AND THE INDICATED PROPORTION OF A HALOGEN SELECTED FROM THE GROUP CONSISTING OF 0.01% TO 0.2% OF ANALYTICALLY DETERMINED CHLORINE, 0.02% TO 0.4% OF ANALYTICALLY DETERMINED BROMINE AND 0.03% TO 0.6% OF ANALYTICALLY DETERMINED IODINE, THE ESSENTIAL CONSTITUENTS TOTALING AT LEAST 85%, WHICH COMPRISES INTRODUCING SILVER IN A CONCENTRATION OF FROM 0.0001% TO 0.3% INTO THE SURFACE OF THE GLASS BY BRINGING THE GLASS AND A FINELY DIVIDED MATERIAL SELECTED FROM THE GROUP CONSISTING OF SILVER AND SILVER COMPOUNDS INTO CONTACT, AND HEATING THE GLASS AND THE SILVER-CONTAINING MATERIAL WHILE THEY ARE IN CONTACT TO EFFECT AN EXCHANGE OF SILVER FOR ALKALI METAL IN THE GLASS THEREAFTER HEATING THE GLASS TO A TEMPERATURE ABOVE ABOUT 550* C. FOR A PERIOD OF AT LEAST 15 MINUTES, COOLING THE GLASS BELOW ABOUT 500* C., AND REHEATING THE GLASS TO A TEMPERATURE NOT LOWER THAN ABOUT 100* C. BELOW ITS SOFTENING POINT UNTIL THE DESIRED DEGREE OF OPACITY IS DEVELOPED. 